With the built-in program called GNOME Disk Utility, you can create an .iso image file back from a bootable USB drive. This feature is important, for example, when you lose the actual .iso file despite you still have the already written USB bootable. This article explains how to re-create such ISO image in Linux Mint and it's also applicable to Ubuntu.

3) Select USB Drive Name

4) Create Disk Image

Click on the menu button > Create Disk Image. This displays dialog asking for the ISO image file name from you and later asking for your password.

5) Determine File Name & Location

Type the name for the ISO image file and its location to your hard drive. In my example, this image is BlankOn X, so I name it blankon-x-tambora-amd64.iso. I choose my Home Documents directory as the destination. Press Start Creating button to start the image creating process.

6) Process Finished

It depends on the size of your USB drive, after processing, you will get the ISO image file on the location you have determined.